This invention relates to optical integrated circuits (OICs) using wavelength division multiplexing (WDM), and more particularly to OICs having wavelength discriminating optical filters with improved crosstalk rejection.
Optical filters are essential components for demultiplexers in WDM optical networks. Recent developments in wavelength discriminating optical filters include filter networks based on multiple path interferometry. These devices use a plurality of substantially uncoupled connecting waveguides between planar multimoded coupling regions. The connecting waveguides and connected coupling regions are constructed on an OIC substrate.
One approach employs a phased array of waveguides connecting two free-propagation regions, and is termed an array waveguide grating filter (AWG). This device is described in U.S. Pat. Nos. 5,002,350 and 5,136,671, issued Mar. 26, 1991 and Aug. 4, 1992 respectively. These patents are incorporated herein by reference for additional details not repeated here.
Another approach uses multimode interference (MMI) coupling regions that are connected by guides to form a generalized Mach-Zehnder Interferometer. See for example, xe2x80x9cNovel InP-based phased array wavelength demultiplexer using a generalized MMI-MZ configurationxe2x80x9d, C. Van Dam et al. Proceedings of the European Conference on Integrated Optics, Genoa, Italy, 1994, pp. 275-278, also incorporated by reference herein.
As the number of channels in the WDM system is increased and/or wavelength spacing between channels is reduced, crosstalk becomes a dominant system issue. While phased waveguide array filters are remarkably effective even in devices with large channels (40 channel WDM devices for example) crosstalk is still a problem.
Sources of crosstalk usually fall into two categories. One is design of the device. There are physical limits to the amount of channel separation that can be geometrically accommodated for a given wafer (OIC) size, so there is a tradeoff between the size of the device and crosstalk tolerance. More difficult sources of crosstalk to address are those arising from material and processing variations. These can be abstruse and unpredictable. Nevertheless, the typical approach to the problem to date has been along these lines, i.e. developing new designs and improving process and material control. See for example, xe2x80x9cThe elimination of sidelobes in the arrayed waveguide WDMxe2x80x9d by S. Day et al., presented at the Integrated Photonics Research Conference, April 29-May 2, 1996, Boston, Mass., and reported in 1996 Technical Digest Series, Vol. 6, of the Optical Society of America, pp. 48-52, ISBNxe2x80x9455752-438-6. While these efforts have met with some success, better solutions for the crosstalk problem in these devices are still sought.
I have developed an OIC approach to crosstalk rejection which deals directly with the problem by providing means in each output channel for filtering unwanted wavelengths. The device structure incorporates a second array of demultiplexers ganged with the first. The second array functions as a clean up filter for the primary demultiplexer. The demultiplexers in the second array are smaller in size than in the first, but otherwise may be essentially identical. This commonality between the filter structures allows the same OIC processing to be employed for making both. In the preferred embodiment of the invention the arrays are AWG arrays.
In an alternative embodiment, a MMI-MZ array is ganged with the primary demultiplexer for clean up filtering. In this case also, common processing steps may be used for fabricating the device.